The present invention relates to precompensation of magnetic transitions written to a medium such as a disk, and more particularly to an apparatus and method for shifting the timing of a magnetic transition to compensate for a timing irregularity occurring due to limited write bubble velocity in a high data rate magnetic recording system.
In magnetic data recording systems such as disk drives, data are recorded on the medium (i.e., magnetic disk) as a series of magnetic field transitions. In many typical systems, a magnetic transition represents a binary xe2x80x9c1xe2x80x9d while the lack of a magnetic transition represents a binary xe2x80x9c0.xe2x80x9d A magnetic field is typically created by passing a current through a write head adjacent to the medium, creating a xe2x80x9cwrite bubblexe2x80x9d which defines a region in which the magnetic field is sufficiently strong to be magnetically recorded on the medium. Magnetic transitions are created by reversing the direction of current flowing through the write head.
The process of reversing the direction of current flowing through the write head requires a finite amount of time, often referred to as the xe2x80x9crise timexe2x80x9d of the write driver employed by the head. The xe2x80x9cwrite bubblexe2x80x9d field created by the head correspondingly contracts as the current is reduced to zero and expands as the current in the opposite direction increases to its steady-state value. The time required for the write bubble to expand to its steady-state dimensions is referred to as the xe2x80x9cflux rise timexe2x80x9d of the head.
In high performance disk drive systems, the data recording rate can be high enough that the write bubble is unable to fully expand to its steady-state dimensions when the data to be recorded requires two or more consecutive magnetic transitions. As a result, the location of the magnetic transition (which is defined by the location of the trailing edge of the write bubble when the write bubble expansion velocity is equal to the linear velocity of the media) is displaced from the ideal location of the transition edge by some non-linear amount. This phenomenon is known as a xe2x80x9cnon-linear transition shiftxe2x80x9d (NLTS) in the magnetic transition pattern. These transition shifts can potentially cause errors in reading data from the disk, effectively limiting the data recording rate of the disk drive to a level at which the magnitude and frequency of occurrence of transition shifts are sufficiently low to ensure accurate data recovery from the disk.
A NLTS in the magnetic transition pattern of a disk drive system may also be caused by the magnetic interaction between the write bubble field and the demagnetization field of nearby magnetic transitions recorded on the disk. This phenomenon has been observed and accounted for in prior art magnetic recording systems by a process known as precompensation. When a current data bit to be recorded requires a magnetic transition, the magnetic recording system examines the bits that were previously recorded. If the previous bits were magnetic transitions, then the timing of the current transition bit is adjusted to ensure that the transition is located properly on the medium, compensating for the effect of the demagnification field of the previous transition bits on the write bubble field used to record the current transition bit. This known precompensation strategy may be referred to as a xe2x80x9clook behindxe2x80x9d precompensation technique, since timing adjustments are made on the basis of the characteristics of previously recorded data bits.
A NLTS that occurs due to a high data recording rate and limited xe2x80x9cflux rise timexe2x80x9d of the write head can only be compensated for by looking at future data bits to be recorded since the location of the first magnetic transition in a series of transitions tends to be affected by this phenomenon. However, there are no existing magnetic recording systems that take this phenomenon into account, and there are no existing magnetic recording systems that employ a xe2x80x9clook aheadxe2x80x9d precompensation technique. Such a technique is the subject of the present invention.
The present invention is a precompensation system that adjusts the timing of magnetic transitions recorded on a medium based on the state of previous data bits recorded on the medium (look-behind precompensation) and on the state of data bits to be subsequently recorded on the medium (look-ahead precompensation). A plurality of data bits are magnetically recorded on a medium (such as a magnetic disk in a disk drive system) by creating a write bubble region encroaching on the medium. The write bubble region has a magnetic polarity that is reversed in a pattern that corresponds to the values of the data bits being recorded on the medium. The timing of the reversing of the magnetic polarity of the write bubble region is adjusted by a precompensation system to ensure that the recorded data bits are properly placed on the medium. The timing adjustment is made by the precompensation system based on a state of at least one data bit previously recorded on the medium and on a state of at least one data bit to be subsequently recorded on the medium.